In recent years, polishing technique has become more and more important in the fields of electronics substrates such as glass substrates for magnetic disks; glass substrates for liquid crystal displays such as thin-film transistor (TFT) type LCDs and twisted nematic (TN) type LCDs; and glass substrates for color filters of liquid crystal television displays and LSI photomasks.
Particularly, in the field of magnetic disk substrates, there is demand for high mechanical performance, particularly high rigidity, so as to reduce the thickness of the substrates in keeping with the trend in weight reduction and to withstand deformation of the disk during high-speed rotation. In addition, there is also strong demand for high recording density. In order to attain high recording density, the flying height of a magnetic head to a magnetic disk substrate has been reduced to a very low height. In order to attain such a low height, the magnetic disk substrates must have high flatness and small surface roughness; i.e., a mirror surface level, and surface defects such as microscratches and micropits must be removed to the utmost. Thus, surface polishing must be performed at high accuracy. In order to meet the aforementioned demands for reduced thickness, high mechanical performance, and high recording density, various improvements have been made to the chemical composition of glass and to the method for producing glass. Specifically, in addition to chemically reinforced glass, there have conventionally been developed, as glass substrates, glass-ceramic. substrates predominantly containing lithium silicate and glass-ceramic substrates containing quartz crystals as a major component. However, these glass substrates have considerably poor processability, and therefore, when a conventional abrasive is used to polish such substrates, processing speed is low, to thereby deteriorate productivity.
Conventionally, in order to perform surface-polishing of a glass substrate, there has been employed an abrasive predominantly comprising a rare earth metal oxide, inter alia, cerium oxide, since cerium oxide exhibits a removal rate several times that of iron oxide, zirconium oxide, or silicon dioxide. During use of such an abrasive, abrasive particles are generally dispersed in liquid such as water. When such an abrasive is used to perform surface-polishing, there are demands for attaining both the aforementioned highly accurate surface polishing performance and a high removal rate.
A variety of measures for increasing the removal rate are disclosed for the case where cerium oxide is used as an abrasive. For example, Japanese Patent Publication (kokoku) No. 38-3643 discloses a polishing method in which colloidal silica, alumina, or a similar substance is added to cerium oxide or a similar substance. Japanese Patent Application laid-Open (kokai) No. 3-146585 discloses an abrasive which comprises cerium oxide as a predominant component and magnesium chloride. However, employment of a sol-form abrasive comprising particles of different species leads to an increase in surface scratches or pits, failing to yield a high-quality surface.
Japanese Patent Application laid-Open (kokai) No. 8-3541 discloses a ceria sol alkaline abrasive containing an organic acid having at least two carboxyl groups for attaining a high-quality surface. Japanese Patent Application laid-Open (kokai) No. 8-41443 discloses a polishing composition containing an abrasive having an average particle size of 0.1-10 μm (2-30 parts by mass) and an alkyl sulfate salt and/or a polyoxyethylene monofatty acid ester (1-20 parts by mass). By employment of such abrasives, highly accurate polishing and polishing performance can be attained simultaneously to a certain degree. However, since a large amount of an organic substance must be employed in addition to abrasive particles, production costs increase and a high-quality surface is difficult to attain, making employment of such abrasives problematic.